The present invention is directed to a method of modifying poly(ethylene oxide). More particularly, the present invention is directed to modified poly(ethylene oxide) that crosslinks upon exposure to moisture and is also melt processable. The present invention also relates to articles made from modified poly(ethylene oxide) that are capable of absorbing relatively large amounts of fluid.
Disposable personal care products, such as pantiliners, diapers, tampons etc., are a great convenience. Such products provide the benefit of one time, sanitary use and are convenient because they are quick and easy to use. However, disposal of such products is a concern due to limited landfill space. Incineration of such products is not desirable because of increasing concerns about air quality and the costs and difficulties associated with separating such products from other disposed, non-incineratable articles. Consequently, there is a need for biodegradable personal care products.
Poly(ethylene oxide) (xe2x80x9cPEOxe2x80x9d) is one of a very few polymers that is both water-soluble and thermally processable. PEO has also been shown to be biodegradable under a variety of conditions. Initial work was done with PEO N-80 (molecular weightxcx9c200,000) which is commercially available from Union Carbide. This grade of PEO is suitable for extrusion processing into film. However, the resultant films have relatively low tensile strength, low ductility, and brittleness. Typical values are 12 MPa break stress and elongation at break of 220%. In an unmodified form, high molecular weight PEO is not thermally processable. Melt fracture and excessive vaporization are observed as PEO is extruded. The resulting resins cannot be cast into thin films and do not have properties that are useful for personal care applications.
A key requirement to achieve a substantially biodegradable personal care product, such as a diaper is to identify and utilize a biodegradable absorbent material that provides the expected levels of leakage protection. There is a wide variety of biodegradable polymers with the potential to become a functional absorbent, but in the current state of development, none provide the leakage protection of sodium polyacrylates. However, sodium polyacrylates are not appreciably degraded in mixed microbial systems unless they are so low in molecular weight (500-700 g/mol) that they are not functional as absorbents.
Consequently, there is a need for biodegradable, absorbent personal care products that are made from materials that can be relatively easily processed, such as by thermal processing, so that it can be easily fabricated into a wide range of structures, such as films, foams, and fibrous webs. Currently available water-soluble resins are not practical for melt processing thin films or fibers for personal care applications. What is needed in the art, therefore, is a water soluble resin that overcomes the difficulties associated with melt processing while also possessing good saline absorption characteristics and functional forms made therefrom are still absorbent, flexible and biodegradable. Examples of water-soluble resins include poly(alkylene oxides) such as PEO, poly(ethylene glycols), block copolymers of ethylene oxide and propylene oxide, poly(vinyl alcohol) or poly(alkyl vinyl ethers).
The present invention is directed to methods for improving saline absorption characteristics of functional forms made from the silane graft modified PEO of the present invention while maintaining the melt processability of silane graft modified PEO as well as the softness and flexibility of personal care products made therefrom. More particularly, the present invention relates to methods of modifying PEO to improve its saline absorption characteristics while retaining its melt processability by grafting organic monomers containing trialkoxy silane functional groups, such as methacryloxypropyl trimethoxy silane, or a moiety that reacts with water to form a silanol group, onto the PEO. The grafting is accomplished by combining PEO, silane-containing monomer(s), an initiator and applying heat. In a preferred embodiment, the method of modification is a reactive-extrusion process. PEOs modified in accordance with this invention have improved water absorption characteristics and melt processabilities and can be thermally processed into films, fibers, foams and other articles which have improved properties over films, fibers, foams and articles similarly processed from unmodified PEO compositions.
To overcome the disadvantages of the prior art, this invention teaches a method of grafting trialkoxy silane functional group-containing organic monomers or monomers containing a moiety that reacts with water to form a silanol group, onto PEO in the melt. Modification of PEO produces a polymer that does not crosslink during melt processing, but rather can be processed into functional forms, such as fibers, films, foams and the like. Yet, when these functional forms made from the modified polymer of the present invention are exposed or subjected to relatively high moisture conditions, they crosslink with each other and form a gel that is capable of absorbing relatively large amounts of saline. Additionally, modified PEO resins in accordance with the present invention can be solidified into pellets for later thermal processing into useful shapes, such as films, fibers, foams and other useful forms which are in turn useful as components in personal care products. The resulting personal care products are soft and flexible and biodegradable.
As used herein, the term xe2x80x9cgraft copolymerxe2x80x9d means a copolymer produced by the combination of two or more chains of constitutionally or configurationally different features, one of which serves as a backbone main chain, and at least one of which is bonded at some point(s) along the backbone and constitutes a side chain. As used herein, the term xe2x80x9cgraftingxe2x80x9d means the forming of a polymer by the bonding of side chains or species at some point(s) along the backbone of a parent polymer. (See Sperling, L.H., Introduction to Physical Polymer Science 1986 pp. 44-47 which is incorporated by reference herein in its entirety.)
Modification of PEO resins with starting molecular weights of between about 3,350 g/mol and 8,000,000 g/mol are useful in the present invention. Modification of PEO resins with starting molecular weights of between about 300,000 g/mol to about 8,000,000 g/mol allows the modified PEO resins to be drawn into films with thicknesses of less than about 0.5 mil. Modification of PEO resins with starting molecular weights of between about 400,000 g/mol to about 8,000,000 g/mol is preferred for fimmaking. Films drawn from the modified PEO compositions have better softness, flexibility, and greater clarity than films drawn from unmodified low molecular weight PEO. Thermal processing of films from high molecular weight PEO modified in accordance with this invention also results in films with improved mechanical properties over films similarly processed from unmodified low molecular weight PEO films.
Modification of PEO resins with starting molecular weights of between about 50,000 g/mol to about 400,000 g/mol allows the modified PEO resins to be extruded into fibers using conventional melt spinning processes. Modification of PEO resins with starting molecular weights of between about 50,000 g/mol to about 200,000 g/mol is preferred for fiber making. The modification of PEO in accordance with this invention improves the melt properties of the PEO allowing the modified PEO to be melted and attenuated into fibers. Thus, the modified PEO can be processed into water-absorbent fibers using both meltblown and spunbond processes which are useful for liners, cloth-like outer covers, etc. in flushable personal products. The modified PEO can be processed into water-absorbent staple fibers for use in bonded, carded webs or in airlaid structures.
These and other features and advantages of the present invention will become apparent after a review of the following detailed description of the disclosed embodiments and the appended claims.
The present invention comprises a grafted PEO that, upon exposure to moisture, crosslinks into a gel structure capable of absorbing relatively large amounts of fluids, such as water or saline. In accordance with the present invention, PEO is graft polymerized with an organic moiety capable of graft polymerization with PEO which moiety contains a trialkoxy silane functional group or which moiety reacts with water to form a silanol group. The silane graft modified PEO resin can be thermally processed into functional forms, such as films, fibers and foams. When these functional forms are exposed to moisture, a crosslinking reaction occurs, by the mechanism shown below, to provide a gel structure capable of absorbing relatively large amounts of water, such as more than 20 grams of saline per gram of polymer under free swell conditions.
Water-soluble polymers useful in the present invention include, but are not limited to, poly(alkylene oxides), such as poly(ethylene oxide) (xe2x80x9cPEOxe2x80x9d), poly(ethylene glycols), block copolymers of ethylene oxide and propylene oxide, poly(vinyl alcohol) and poly(alkyl vinyl ethers). These water-soluble polymers must be capable of graft polymerization with an organic moiety containing a trialkoxy silane functional group or a moiety that reacts with water to form a silanol group. The preferred water-soluble polymer for use in the present invention is PEO. The process for the graft polymerization of PEO with methacryloxypropyl trialkoxy silane followed by cross-linking upon exposure to moisture is shown below.

Since crosslinking of the silane graft modified PEO does not normally occur during thermal processing, the graft modified PEO of the present invention provides for more robust thermal processing into functional forms. Furthermore, since the process of forming the silane graft modified PEO of the present invention does not require the use of aqueous solutions, there are no costly and time consuming evaporation steps involved.
The PEO resins useful for graft modification in accordance with the present invention include, but are not limited to, PEO resins having initial reported approximate molecular weights ranging from about 10,000 g/mol to about 8,000,000 g/mol as determined by rheological measurements. Such PEO resins are commercially available from, for example, Union Carbide Corporation having offices in Danbury, Conn., and are sold under the trade designations POLYOX(copyright) 205, POLYOX(copyright) N-10, POLYOX(copyright) N-80, POLYOX(copyright) WSR N-750, POLYOX(copyright) WSR N-12K and POLYOX(copyright) UCARFLOC(copyright) Polymer 309.
Fibers, films and foams can be made using conventional processing methods from commercially available PEO resins when modified in accordance with this invention. The PEO resins useful for modification for fiber-making purposes include, but are not limited to, PEO resins having initial reported approximate molecular weights ranging from about 50,000 g/mol to about 400,000 g/mol. Higher molecular weights are desired for increased mechanical and physical properties and lower molecular weights are desired for ease of processing. Desirable PEO resins for fiber making have molecular weights ranging from 50,000 to 300,000 g/mol before modification and more desired PEO resins for fiber making have molecular weights ranging from 50,000 to 200,000 g/mol before modification. The PEO compositions modified from PEO resins within the above resins provide desirable balances between mechanical and physical properties and processing properties. Three PEO resins within the above preferred ranges are commercially available from Union Carbide Corporation and are sold under the trade designations POLYOX(copyright) N-750, POLYOX(copyright) WSR N-10 and POLYOX(copyright) WSR N-80. These three resins have reported approximate molecular weights, as determined by rheological measurements, of about 100,000 g/mol to 300,000 g/mol.
Other PEO resins available from, for example, Union Carbide Corporation, within the above approximate molecular weight ranges are sold under the trade designations WSR N-750, WSR N-3000, WSR-3333, WSR-205, WSR-N-12K, WSR-N-60K, WSR-301, WSR Coagulant, WSR-303. (See POLYOX(copyright): Water Soluble Resins, Union Carbide Chemicals and Plastic Company, Inc., 1991 which is incorporated by reference herein in its entirety.) Both PEO powder and pellets of PEO can be used in this invention since the physical form of PEO does not affect its behavior in the melt state for grafting reactions. This invention has been demonstrated by the use of PEO in powder form as supplied by Union Carbide. However, the PEO resins to be modified may be obtained from other suppliers and in other forms, such as pellets. The PEO resins and modified compositions may optionally contain various additives, such as, plasticizers, processing aids, rheology modifiers, antioxidants, UV light stabilizers, pigments, colorants, slip additives, antiblock agents, etc., which may be added before or after modification.
Organic monomers capable of graft polymerization with PEO which monomers contain a trialkoxy silane functional group or a moiety that reacts with water to form a silanol group are useful in the practice of this invention. The trialkoxy silane functional group has the following structure: 
wherein R1, R2 and R3 are alkyl groups independently having 1 to 6 carbon atoms. The term xe2x80x9cmonomer(s)xe2x80x9d as used herein includes monomers, oligomers, polymers, mixtures of monomers, oligomers and/or polymers, and any other reactive chemical species which is capable of covalent bonding with the parent polymer, PEO. Ethylenically unsaturated monomers containing a trialkoxy silane functional group are appropriate for this invention and are desired. Desired ethylenically unsaturated monomers include acrylates and methacrylates. A particularly desirable ethylenically unsaturated monomer containing a trialkoxy silane functional group is methacryloxypropyl trimethoxy silane. Methacryloxypropyl trimethoxy silane is commercially available from Dow Corning, having offices in Midland, Mich., under the trade designation Z-6030 Silane. Other suitable ethylenically unsaturated monomers containing a trialkoxy silane functional group include, but are not limited to, methacryloxyethyl trimethoxy silane, methacryloxypropyl triethoxy silane, methacryloxypropyl tripropoxy silane, acryloxypropylmethyl dimethoxy silane, 3-acryloxypropyl trimethoxy silane, 3-methacryloxypropylmethyl diethoxy silane, 3-methacryloxypropylmethyl dimethoxy silane, and 3-methacryloxypropyl tris(methoxyethoxy) silane. However, it is contemplated that a wide range of vinyl and acrylic monomers having trialkoxy silane functional groups or a moiety that reacts easily with water to form a silanol group, such as a chlorosilane or an acetoxysilane, provide the desired effects to PEO and are effective monomers for grafting in accordance with the present invention.
The amount of organic monomer having trialkoxy silane functional groups or silanol-forming functional groups relative to the amount of PEO may range from about 0.1 to about 20 weight percent of monomer to the weight of PEO. Desirably, the amount of monomer should exceed 0.1 weight percent in order sufficiently to improve the processability of the PEO. A range of grafting levels is demonstrated in the Examples. Typically, the monomer addition levels are between about 1.0% and about 15% of the weight of the base PEO resin; particularly, between about 1.0% and about 10% of the weight of the base PEO resin; especially, between about 1.5% and about 5.5% of the weight of the base PEO resin.
A variety of initiators may be useful in the practice of this invention. When grafting is achieved by the application of heat, as in a reactive-extrusion process, it is desirable that the initiator generates free radicals through the application of heat. Such initiators are generally referred to as thermal initiators. For the initiator to function as a useful source of radicals for grafting, the initiator should be commercially and readily available, stable at ambient or refrigerated conditions, and generate radicals at reactive-extrusion temperatures.
Compounds containing an Oxe2x80x94O, Sxe2x80x94S, or Nxe2x95x90N bond may be used as thermal initiators. Compounds containing Oxe2x80x94O bonds; i.e., peroxides, are commonly used as initiators for graft polymerization. Such commonly used peroxide initiators include: alkyl, dialkyl, diaryl and arylalkyl peroxides such as cumyl peroxide, t-butyl peroxide, di-t-butyl peroxide, dicumyl peroxide, cumyl butyl peroxide, 1,1-di-t-butyl peroxy-3,5,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexyne-3 and bis(a-t-butyl peroxyisopropylbenzene); acyl peroxides such as acetyl peroxides and benzoyl peroxides; hydroperoxides such as cumyl hydroperoxide, t-butyl hydroperoxide, p-methane hydroperoxide, pinane hydroperoxide and cumene hydroperoxide; peresters or peroxyesters such as t-butyl peroxypivalate, t-butyl peroctoate, t-butyl perbenzoate, 2,5-dimethylhexyl-2,5-di(perbenzoate) and t-butyl di(perphthalate); alkylsulfonyl peroxides; dialkyl peroxymonocarbonates; dialkyl peroxydicarbonates; diperoxyketals; ketone peroxides such as cyclohexanone peroxide and methyl ethyl ketone peroxide. Additionally, azo compounds such as 2,2xe2x80x2-azobisisobutyronitrile abbreviated as AIBN, 2,2xe2x80x2-azobis(2,4-dimethylpentanenitrile) and 1,1xe2x80x2-azobis(cyclohexanecarbonitrile) may be used as the initiator. This invention has been demonstrated in the following Examples by the use of a liquid, organic peroxide initiator available from R. T. Vanderbilt Company, Inc. of Norwalk, Conn., sold under the trade designation VAROX DBPH peroxide which is a free radical initiator and comprises 2,5-bis(tert butylperoxy)-2,5-dimethyl hexane along with smaller amounts of di(tert butylperoxide). Other initiators may also be used, such as LUPERSOL(copyright) 101 and LUPERSOL(copyright) 130 available from Elf Atochem North America, Inc. of Philadelphia, Pa.
A variety of reaction vessels may be useful in the practice of this invention. The modification of the PEO can be performed in any vessel as long as the necessary mixing of PEO, the monomer and the initiator is achieved and enough thermal energy is provided to affect grafting. Desirably, such vessels include any suitable mixing device, such as Brabender Plasticorders, Haake extruders, Bandbury mixers, single or multiple screw extruders, or any other mechanical mixing devices which can be used to mix, compound, process or fabricate polymers. In a desired embodiment, the reaction device is a counter-rotating twin-screw extruder, such as a Haake extruder available from Haake, 53 West Century Road, Paramus, N.J. 07652 or a co-rotating, twin-screw extruder, such as a ZSK-30 twin-screw, compounding extruder manufactured by Werner and Pfleiderer Corporation of Ramsey, N.J. It should be noted that a variety of extruders may be used to modify the PEO in accordance with the invention provided that mixing and heating occur.
The present invention is further illustrated by the following examples, which are not to be construed in any way as imposing limitations upon the scope thereof. On the contrary, it is to be clearly understood that resort may be had to various other embodiments, modifications, and equivalents thereof which, after reading the description herein, may suggest themselves to those skilled in the art without departing from the spirit of the present invention and/or the scope of the appended claims.